1. Field of the Invention
The present invention relates to a screen printing machine and particularly to the art of preventing a defective printing caused by clogging of the screen.
2. Related Art Statement
A screen printing machine is used in a wide range of art. For example, a screen printing machine is used for applying an adhesive or a solder paste to a printed wiring board (xe2x80x9cPWBxe2x80x9d) before electric components are mounted on the PWB to produce one or more electric circuits on the PWB. This screen printing machine includes a screen which has a plurality of openings and on which a printing material such as an adhesive or a solder paste is placed, and a squeegee for forcing the printing material into the openings of the screen and thereby printing the material through the openings onto predetermined places on a PWB which is positioned below the screen. Thus, the printing material is printed on the PWB, at respective places thereon corresponding to the openings of the screen, and with respective shapes and volumes corresponding to respective shapes and capacities of the openings. However, if one or all of the openings of the screen is or are clogged (hereinafter, referred to simply as xe2x80x9cthe clogging of the screenxe2x80x9d), only an insufficient amount of printing material or substantially no printing material is printed on the PWB.
In order to avoid the above-identified problem, a screen printing machine for printing a solder paste on a PWB has been used in such a way that an image taking device is employed to take an image of the solder paste printed on the PWB and image data representing the taken image is used to estimate about the clogging of the screen. If the clogging of the screen is estimated, or an early sign of the clogging is found, the screen is cleaned, or replaced with another screen. However, for some reasons, in particular, the reason that the color and lightness of surfaces of the PWB are near to those of the solder paste, it is not easy to take, with an image taking device, an image of the solder paste printed on the PWB. In addition, it is not easy to determine or measure the shape and/or size (three-dimensional, in particular) of the solder paste printed on the PWB. Moreover, there are some cases where the shape and/or size of the solder paste printed on the PWB may not accurately reflect the clogging of the screen. Thus, even though the screen is actually clogged, the clogging of the screen may be overlooked, which may result in producing one or more defective electric circuits on the PCB.
While the case where the solder paste is printed on the PWB has been described as an example, the above-identified problem may occur to not only the case where an adhesive is printed on a PWB but also the case where a printing material is printed on an object other than a PWB.
The present invention provides a screen inspecting method, a screen inspecting apparatus, and a screen printing machine that have one or more of the technical features that are described below in respective paragraphs given parenthesized sequential numbers (1) to (11). Any technical feature that includes another technical feature shall do so by referring, at the beginning, to the parenthesized sequential number given to the latter technical feature. Thus, two or more of the following technical features may be combined, if appropriate. Each technical feature may be accompanied by a supplemental explanation, as needed. However, the following features and the appropriate combinations thereof are just examples to which the scope of the present invention is by no means limited.
(1) According to a first feature of the present invention, there is provided a method of inspecting a screen of a screen printing machine, comprising the steps of taking, with an image taking device, an image of at least one opening of the screen, and judging, based on image data representing the taken image, whether the opening of the screen is clogged. The image taking device may take an image or respective images of only one or more pre-selected openings of the screen. For example, the image taking device may be used to take an image of such an opening that is likely clogged or must be prevented from being clogged. Otherwise, the image taking device may take respective images of all the openings of the screen. In the latter case, the image taking device may take the respective images of all the openings, all at once, or alternatively may divide the openings into a plurality of groups and sequentially take respective images of the openings of each of the groups. In a special manner belonging to the alternative case, the image taking device is used to take sequentially an image of each of the openings. Respective images of openings of a single screen may be taken by a single image taking device or the cooperation of a plurality of image taking devices. In the case where the image taking device takes an image of an entire opening of the screen, it can be judged, based on image data representing the taken image, whether at least a portion of the opening of the screen is clogged with the printing material. In addition, the present screen inspecting method can be used, as needed, to identify which portion of an opening of the screen is clogged to which degree. Thus, in the case where the present method is carried out, as needed, during a printing operation in which a printing material is sequentially printed on each of a plurality of objects, the clogging of the screen or an early sign of the clogging can be detected, and an operator can take a necessary action, e.g., cleaning the screen or replacing the screen with another screens. Meanwhile, in the case where the present method is carried out each time printing on each object terminates, and if clogging is detected for an opening for which no clogging had been detected at the previous time, it can be estimated that a defective printing would have occurred to the previous object.
(2) According to a second feature of the present invention that includes the first feature (1), the step of taking the image comprises lighting the screen on one of both sides of the screen that is opposite to the other side on which the image taking device is provided, and taking a silhouette image of the opening. Since a silhouette image is not influenced by the color and/or lightness of the screen and/or the printing material, it can be accurately judged, based on image data representing the silhouette image, whether the opening of the screen is clogged. Since generally a solder paste is opaque, it is advantageous to take a silhouette image of the opening. In the last case, the screen may be a transparent one.
(3) According to a third feature of the present invention that includes the first or second feature (1) or (2), the step of judging comprises comparing the image data representing the taken image, with reference image data representing an image of the opening without clogging. It is easy to identify clogging of the opening, by comparing actual image data representing the taken image of the opening, with reference image data representing an image of the opening without clogging. In addition, which portion of the opening is clogged to which degree can be identified by comparing the shapes and size of the opening that are represented by the actual image data, with those represented by the reference image data. Moreover, irrespective of the shape of a portion of the opening through which a light passes, clogging of the opening can be identified by comparing the area of the opening represented by the actual image data, with that represented by the reference image data. In the last case, each of the actual image data and the reference image data may consist of only area data representing an area of the opening.
(4) According to a fourth feature of the present invention that includes any one of the first to third features (1) to (3), the step of taking the image comprises taking a first image of the opening before at least one printing of the screen printing machine, and taking a second image of the opening after the printing. Whether one or more printings of the screen printing machine has or have caused clogging of one or more openings of the screen can be identified by comparing the first image or images of the opening or openings before the printing or printings, with the second image or images of the opening or openings after the printing or printings. For example, image data representing an image of a screen which apparently has no clogged openings, e.g., a screen which has just been cleaned, or a new screen, may be employed as reference image data, and compared with actual image data representing an actual image of the screen taken after one or more printings. In this case, whether the printing or printings has or have caused clogging of one or more openings of the screen can be identified.
(5) According to a fifth feature of the present invention that includes any one of the first to third features (1) to (3), the step of taking the image comprises taking a first image of the opening before each of a plurality of printings of the screen printing machine, and taking a second image of the opening after said each printing. Since the image taking device takes a first image of the opening before each of a plurality of printings and a second image of the opening after the each printing, it can be judged whether the each printing is defective. If the first image indicates that the screen has no clogged openings but the second image indicates that the screen has one or more clogged openings, it can be estimated that a defective printing of the printing material would have occurred to the object.
(6) According to a sixth feature of the present invention, there is provided an apparatus for inspecting a screen of a screen printing machine, comprising an opening-image taking device which takes an image of at least one opening of the screen; and an image-data processing device which judges, based on image data representing the image taken by the opening-image taking device, whether the opening of the screen is clogged. The present screen inspecting apparatus can carry out the screen inspecting method according to the first feature (1). Each of the third to fifth features (3) to (5) may be employed by the present screen inspecting apparatus.
(7) According to a seventh feature of the present invention that includes the sixth feature (6), the screen inspecting apparatus further comprises a lighting device which lights the screen on one of both sides of the screen that is opposite to the other side on which the opening-image taking device is provided, so that the opening-image taking device takes a silhouette image of the opening.
(8) According to an eighth feature of the present invention that includes the sixth feature (6), the screen inspecting apparatus further comprises an opening-image-taking-device moving device which moves the opening-image taking device to an arbitrary position on a plane parallel to a plane of the screen. In this case, even if the opening-image taking device may be one whose field of view is considerably narrow, the present screen inspecting apparatus can reliably inspect clogging of a wide screen.
(9) According to a ninth feature of the present invention that includes the eighth feature (8), the screen inspecting apparatus further comprises a lighting plate which is provided on one of both sides of the screen that is opposite to the other side on which the opening-image taking device is provided, such that the lighting plate extends substantially parallel to the screen, and which emits, from a lighting surface thereof facing a substantially entire area of the screen, a light toward the screen, so that the opening-image taking device takes a silhouette image of the opening. The employment of the lighting plate according to this feature contributes to reducing the production cost of the present screen inspecting apparatus as a whole. However, in the case where the opening-image taking device is moved by an opening-image-taking-device moving device, a lighting-device moving device may be employed to move a lighting device such that the lighting device continues to be opposed to the opening-image taking device. In the latter case, the lighting device may be a small-size one.
(10) According to a tenth feature of the present invention, there is provided a screen printing machine, comprising a supporting device having a supporting surface on which an object is supported; a screen which extends parallel to the supporting surface of the supporting device and which has a plurality of openings; a squeegee which is slid on the screen to print a printing material placed on the screen, onto the object supported by the supporting device, through the openings of the screen; an elevating and lowering device which elevates and lowers one of the screen and the supporting device, so that said one of the screen and the supporting device is moved toward, and away from, the other of the screen and the supporting device; an opening-image taking device which takes an image of at least one of the openings of the screen; an opening-image-taking-device moving device which moves, in a space which is produced between the screen and the supporting device which are moved away from each other by the elevating and lowering device, the opening-image taking device to an arbitrary position on a plane parallel to a plane of the screen, so that the image taking device takes the image of said one opening of the screen; and an image-data processing device which judges, based on image data representing the image taken by the opening-image taking device, whether said one opening of the screen is clogged. The present screen printing machine has the function of inspecting clogging of the screen. The ninth feature (9) may be employed by the present screen printing machine.
(11) According to an eleventh feature of the present invention that includes the tenth feature (10), the screen printing machine further comprises a reference-mark-image taking device including a first image taking portion which takes respective images of a plurality of first reference marks provided on a lower surface of the screen, and a second image taking portion which takes respective images of a plurality of second reference marks provided on an upper surface of the object supported by the supporting device; a reference-mark-image-taking-device moving device which moves the reference-mark-image taking device to each of a plurality of positions where the reference-mark-image taking device is opposed to a corresponding one of the first reference marks and the second reference marks; and a positioning device which positions, based on image data representing the images taken by the reference-mark-image taking device, the object and the screen relative to each other in a direction parallel to the screen, and at least the first image taking portion of the reference-mark-image taking device is provided by the opening-image taking device. The reference-mark-image taking device and the reference-mark-image-taking-device moving device have been used in the art. In the case where at least a portion of the reference-mark-image taking device and the reference-mark-image-taking-device moving device is also used as at least a portion of the opening-image taking device and the opening-image-taking-device moving device, the present screen printing machine can enjoy the function of inspecting clogging of the screen while minimizing the increase of the production cost.